Add Pathfinder 2e game system mode

Implements PF2e as an alternative game system alongside D&D 5e/5.5e.
Settings modal "Game System" selector switches conditions, bestiary,
stat block layout, and initiative calculation between systems.

- Valued conditions with increment/decrement UX (Clumsy 2, Frightened 3)
- 2,502 PF2e creatures from bundled search index (77 sources)
- PF2e stat block: level, traits, Perception, Fort/Ref/Will, ability mods
- Perception-based initiative rolling
- System-scoped source cache (D&D and PF2e sources don't collide)
- Backwards-compatible condition rehydration (ConditionId[] → ConditionEntry[])
- Difficulty indicator hidden in PF2e mode (excluded from MVP)

Closes dostulata/initiative#19

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/domain/src/__tests__/initiative.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 import {
 	calculateInitiative,
+	calculatePf2eInitiative,
 	formatInitiativeModifier,
 } from "../initiative.js";
 
@@ -93,6 +94,26 @@ describe("calculateInitiative", () => {
 	});
 });
 
+describe("calculatePf2eInitiative", () => {
+	it("returns perception as both modifier and passive", () => {
+		const result = calculatePf2eInitiative(11);
+		expect(result.modifier).toBe(11);
+		expect(result.passive).toBe(11);
+	});
+
+	it("handles zero perception", () => {
+		const result = calculatePf2eInitiative(0);
+		expect(result.modifier).toBe(0);
+		expect(result.passive).toBe(0);
+	});
+
+	it("handles negative perception", () => {
+		const result = calculatePf2eInitiative(-2);
+		expect(result.modifier).toBe(-2);
+		expect(result.passive).toBe(-2);
+	});
+});
+
 describe("formatInitiativeModifier", () => {
 	it("formats positive modifier with plus sign", () => {
 		expect(formatInitiativeModifier(7)).toBe("+7");